Fluoropolymers have been used in a variety of applications because of their desirable properties such as heat resistance, chemical resistance, weatherability, UV-stability etc. Various applications of fluoropolymers are described, for example, in “Modern Fluoropolymers”, edited by John Scheirs, Wiley Science 1997. Fluoropolymers include homopolymers and copolymers of a fluorinated or non-fluorinated olefin.
Fluoropolymers include melt-processible and non-melt-processible polymers. For example, polytetrafluoroethylene (PTFE) and copolymers of tetrafluoroethylene (TFE) with small amounts (for example, not more than 0.5% by weight) of a comonomer are generally not melt-processible with conventional equipment used for the processing of thermoplastic polymers because of their high molecular weights and high melt viscosities (about 107 Pa*s at a melting point of about 380° C.). Melt-processible fluoropolymers can be obtained from various fluorinated monomers and/or combinations of fluorinated and non-fluorinated monomers. Depending on the monomers used in the preparation melt-processible fluoropolymers may be perfluorinated or partially fluorinated. Melt-processible polymers can be processed with equipment typically used for the processing of thermoplastic polymers, such as molding, injection molding, coating or extrusion.
The rate of extrusion of melt-processible fluoropolymers is limited to the speed at which the polymer melt undergoes melt fracture. This may be important in thermoforming processes such as wire and cable extrusion, film extrusion, blown film extrusion, injection molding etc. If the rate of extrusion exceeds the rate at which melt fracture occurs (known as critical shear rate), an undesired rough surface of the extruded article may be obtained.
Using an extrusion die with a relatively large orifice and then drawing the extruded melt to the desired final diameter may increase the process rate of the melt processible fluoropolymer. Herein, the melt draw is commonly characterized by the draw down ratio calculated as the ratio of the cross-sectional area of the die opening to the ratio of the cross-sectional area of the finished extrudate. To obtain a high draw down ratio, for example, in the order of up to 100, the polymer melt should exhibit a sufficiently high elongational viscosity. Otherwise the cone stability of the polymer melt in the extrusion will be insufficient, which results in undesired diameter variations of the extruded article as well as frequent cone-breaks.
Accordingly, there exists a continuous need for fluoropolymers that can be melt-processed at higher shear rates and that have a high elongational viscosity. Various attempts have been made in the art to obtain such fluoropolymers or to obtain fluoropolymers that can be faster processed.
A known approach in the art is to substantially broaden the molecular weight distribution of the fluoropolymer thereby increasing the shear rate. However, the gain in critical shear rate is usually to the expense of weaker overall mechanical properties.
An alternative approach is to tailor the topography of the fluoropolymers by using specific modifier molecules leading to the formation of branched rather than linear polymers. This approach has been described, for example, in WO2004/094491, WO2006/031316 and U.S. Pat. No. 4,612,357. These modifiers are typically olefins containing one or more halogen, typically bromine or iodine, which can be easily detached from the chain during radical polymerization. However, the formation of bromine or iodine containing products may require subsequent process steps to further stabilize the polymer.
In JP 2002/012626 aliphatic perfluorinated divinyl ethers have been described as modifiers generating long chain branches. However, aliphatic vinyl ether chains have been found to be labile when submitted to high pH environments. Therefore, precautions have to be taken in down stream processes and work up proceedings. Additionally, aliphatic divinyl ethers are rather expensive raw materials leading to increased process costs.